Methicillin resistant Staphylococcus aureus (MRSA) has emerged as a major public health threat and the expansion worldwide of a single clone (USA300) associated with life threatening infections even in immunocompetent adults has re-focused interest in developing novel therapeutics to treat this infection. One strategy emerging to treat antibiotic resistant infections is to develop drugs that target virulence but not bacterial growth. This approach is postulated to limit the development of resistance while enhancing host defense by permitting immune effectors to kill and clear the pathogenic bacteria rendered avirulent by the drug. Whereas this strategy has had success in a few animal models of primarily gram-negative bacterial infection, it has not been pursued for treatment of MRSA infection. The virulence factors identified to date as essential for invasive MRSA infection are globally regulated in part by a quorum sensing operon, agr Importantly, our recent work identifying apolipoprotein B as an innate barrier that antagonizes agr signaling demonstrates that host defense against invasive infection can be accomplished by blocking agr signaling. Therefore, we propose to use small molecule inhibitors (SMIs) identified by screening >50,000 compounds for antagonism of agr to treat experimental MRSA infections. A significant barrier in translating in vitro screening results to successful treatment of infection in vivo is ascertaining an appropriate vehicle for solubilizing these largely hydrophobic compounds while maintaining pharmacologic utility (9). In preliminary data we show that two SMIs complexed with cyclodextrin as a vehicle demonstrate excellent in vivo therapeutic efficacy against quorum sensing dependent MRSA infection. The goal of this Merit Review is to test the hypothesis that small molecule inhibitors complexed with cyclodextrin that attenuate virulence in vitro will enhance host defense against MRSA infection in vivo. To test this hypothesis, we will pursue the following specific aims: 1) To determine the virulence mechanisms affected by small molecule inhibitors complexed with cyclodextrin in both agr+ and Dagr USA300 and USA400 S. aureus strains; 2) To determine in multiple models of agr+ and Dagr S. aureus infection the efficacy and potency of small molecule inhibitors complexed with cyclodextrin, including synergism with existing antibiotics; and 3) To determine the efficacy of innate immune effectors in killing and clearance of agr+ and Dagr USA300 and USA400 S. aureus strains treated with small molecule inhibitors complexed with cyclodextrin.